A background art is explained with reference to a hybrid shovel as an example.
The hybrid shovel includes an engine functioning as a power source, a generator motor and a hydraulic pump connected to the engine, and a power storage unit charged by power generation operation of the generator motor. In the hybrid shovel, the generator motor timely operates as a motor with electric power of the power storage unit and assists the engine.
In the hybrid shovel, the capacity of the power storage unit decreases when temperature is low. Power storage performance (discharge performance) is deteriorated and sufficient electric power cannot be obtained. Therefore, in the hybrid shovel, in a low-temperature environment in winter and the like, it is desired to take warming-up measures for heating the power storage unit to appropriate temperature like the engine.
As a warming-up technique for the power storage unit, Japanese Unexamined Patent Publication No. 2013-52866 is publicly known.
In Japanese Unexamined Patent Publication No. 2013-52866, when the temperature of the power storage unit drops to a setting value, which indicates a warming-up start temperature, or less, the generator motor forcedly charges and discharges the power storage unit and raises the temperature of the power storage unit with internal heating.
On the other hand, Japanese Unexamined Patent Publication No. H11-117914 discloses a technique for, in order to raise the temperature of hydraulic oil in a short time and with less noise, actuating a hydraulic load (a relief valve) to increase a pump load (=an engine load).
In the case of the hybrid shovel, the generator motor is a load of the engine in addition to the hydraulic pump. Therefore, as explained in Japanese Unexamined Patent Publication No. 2013-52866, when the generator motor is caused to perform the power generation operation to warm up the power storage unit, a load of the engine of the hybrid shovel is basically large compared with a load of the engine of the hydraulic shovel. Note that the power storage unit is charged when the generator motor performs the power generation operation.
However, in Japanese Unexamined Patent Publication No. 2013-52866, since warming-up of the hydraulic oil is performed by applying a hydraulic load simultaneously with the warming-up of the power storage unit, it is likely that the load of the engine becomes excessively large and an engine stall occurs.
Since Japanese Unexamined Patent Publication No. H11-117914 relates to the technique for raising the temperature of the hydraulic oil, there is no disclosure concerning warming up of the power storage unit in Japanese Unexamined Patent Publication No. H11-117914.